Biological processes to treat contaminated water take many forms. Generally these involve exposure of the waste stream to one or more forms of microorganism that stabilize or digest the contaminants. The microorganisms are chosen to complement the waste stream both in terms of sewage contents and chemical environment, since any species of microorganism favors a particular environment with limited tolerance for variation. For example, the activated sludge process utilizes aerobic bacteria that remove the soluble biological oxygen demand (BOD) from wastewater. This generally involves conducting wastewater into an aeration basin containing a suspension of digestive microorganisms, thereby forming a “mixed liquor” that is aerated to furnish oxygen for respiration of biomass; the biomass sorbs, assimilates and metabolizes the BOD of the wastewater. After a suitable period of aeration, the mixed liquor is introduced into a clarifier in which the biomass settles, allowing the treated wastewater to overflow into an outlet effluent stream.
An aspect of traditional wastewater treatment is adequate agitation of the mixed liquor in order to speed contact between the digestive microorganisms and waste materials, which can be suspended or dissolved in the wastewater. An optimal amount of turbulence is generally dictated more by economics than by process requirements; high agitation rates are theoretically the most desirable, but are also expensive to attain.
An exception to this practice involves the use of fixed-growth media, where the biological organisms are maintained on fixed supports rather than dispersed in suspension. In this case mixing is avoided to prevent shear that might remove the biological attached growth. The application of fixed-growth systems is ordinarily restricted to soluble, non-particulate contaminants; in addition, these processes are limited in loading capacity by the surface area of the biological support and the diffusion characteristics of the waste stream.
Fluidized-bed systems represent a combination of suspension and fixed-growth processes, but require added media for surface area, mixing sufficient to maintain homogeneity of the media and its attached biological growth, and periodic or continuous removal of the media for regeneration.
All of these systems ordinarily are limited to one category of microorganism, since differing biological processes vary significantly in terms of multiplication rates, optimum conditions, and preferred inputs and waste products. Most generally, microorganisms for wastewater treatment include aerobic, anaerobic and anoxic species, all of which are sustained by very different (and mutually inconsistent) environments. Process conditions can also restrict the applicability of a particular biological approach. For example, the optimal biological process for a particular wastewater composition might require a longer solids retention time than that afforded by economically feasible complete-mix processes, and exhibit greater throughput needs than can be met with fixed-film and fluid-bed film reactors.
Wastewater streams often contain large amounts of free oils and greases which can have an adverse effect on the components of wastewater systems When fed into wastewater systems which employ biological processes, free oils and greases can diminish the ability of biological processes to remove and stabilize pollutants.
The effect of free oils and greases which are present in wastewater streams is particularly significant in wastewater purification processes carried out by combination bioreactor/membrane devices employing one discrete component which uses biological treatment processes to remove biological nutrients and another discrete component comprising immersed hollow fibre polymeric membranes systems to remove suspended solids.
Aerobic processes create a scum layer of grease, oils and suspended. An accumulation of grease in the biological component of the process can inhibit oxygen transfer efficiency by coating mechanical equipment such as air diffusers and by diminishing the formation of activated sludge flocs and adversely influences their consequent ability to absorb oxygen. Biological nutrient removal efficiency is dramatically affected by the ability of the process to transfer oxygen to aerobic process microorganisms.
Free oils and grease also inhibit the membrane filtration component of the process because they can adhere to immersed hollow fiber membranes, thereby reducing their permeability and ability to filter the water stream emerging from the biological portion. The resultant accumulation of free oils and greases will negatively impact the ability of the wastewater process to effectively treat wastewater containing nutrients.